Which of these numbers is prime? ${67,\ 69,\ 78,\ 87,\ 93}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 67 are 1 and 67. The factors of 69 are 1, 3, 23, and 69. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 87 are 1, 3, 29, and 87. The factors of 93 are 1, 3, 31, and 93. Thus, 67 is a prime number.